Method and device for recognition of natural skin

ABSTRACT

The invention relates to a method for the recognition of natural skin ( 5 ), wherein the skin surface ( 4 ) is illuminated at an irradiation point ( 1 ) with light from the visible spectrum or the adjacent spectrum, wherein that part of the light entering through the skin surface ( 4 ) at the irradiation point ( 1 ), scattered in the skin ( 5 ), and exiting from the skin surface ( 4 ) again, is detected at a detection point ( 9 ), using a detector ( 20 ), and wherein the signal determined by the detector ( 20 ) is passed to a comparator and compared with stored data. Furthermore, a device for carrying out the method is an object of the invention.

The invention relates to a method for the recognition of natural skin,as well as to a device for carrying out the method.

In the checking of authorization with regard to access to closedsystems, which can be represented by room complexes or data networks,for example, biometric checks are being performed, to an increasingdegree, for identification of the person authorized for access, sincethere is no risk that the individual biometric characteristics of anindividual can be lost or passed on, as is the case with passwords orkeys. In practice, the use of the papillary line patterns of the palmsor the fingertips, as a biometric characteristic that is different forevery individual and does not change, has proven itself, whereby methodsare known with which these papillary line patterns can be opticallydetected in contact-free manner. In the case of these measurements, thepapillary line pattern is evaluated as such, in other words the topologyof the skin surface or, to state it differently, the pattern of therelief formed by the papillary ridges, is evaluated. In this connection,there is the problem that when the papillary line pattern is known, themeasurement apparatus for detecting the biometric data can, inprinciple, be deceived by means of false copies that contain areproduction of the papillary line pattern in accordance with thethree-dimensional original, and which are possible, for example, in theform of two-dimensional reproductions, three-dimensional reproductions,or coverings on natural fingers.

The invention is based on the task of indicating a method with which thesecurity of biometric methods for checking access authorization,particularly those using the evaluation of papillary line patterns, canbe improved to prevent attempts at deception. It is furthermore the taskof the invention to make available a device with which this method canbe carried out.

This task is accomplished, with regard to the part of the invention thatrelates to the method, by means of a method for the recognition ofnatural skin, wherein the skin surface is illuminated at an irradiationpoint with light from the visible spectrum or the adjacent spectrum,wherein that part of the light entering through the skin surface at theirradiation point, scattered in the skin, and exiting from the skinsurface again, is detected at a detection point, using a detector, andwherein the signal determined by the detector is passed to a comparatorand compared with stored data.

This method offers the advantage that optical properties of the naturalskin are checked by means of measuring the scattered light, therebyprecluding a large number of possibilities of deception of the typestated initially. For this purpose, the skin is illuminated, at theirradiation point, with a beam of light that is diffusely reflected atthe surface, in part. The remaining part of the beam of light penetratesinto the tissue of the skin, and is distributed in this volume by meansof multiple scattering, whereby a fraction of this scattered light exitsfrom the skin surface again and thereby makes the skin appear bright.Since natural skin is composed of complex organic structures andcontains a large number of optically active substances, its opticalproperties, particularly with regard to scattering (Mie and Rayleighscattering) and the absorption that can be determined using scatteredlight cannot be imitated, or can be imitated only with great effort. Itis advantageous in this connection if the detection point is located ata different location from the irradiation point.

The security of the method according to the invention can be furtherincreased in that light from a limited spectrum range is used forilluminating the irradiation point, since this range can be coordinatedwith the optical properties, with regard to scattering and absorption,of substances that are present in the skin, in other words, light from aspectrum range in which the absorption and scattering of a naturalsubstance that naturally occurs in the skin assume characteristic valuesis used for illuminating the irradiation point.

In order to improve the measurement result, it is furthermore provided,within the scope of the invention, that several limited spectrum rangesare used for illuminating the irradiation point. It is advantageous, inthis connection, if light from the spectrum ranges around 600 nm andaround 800 nm is used for illuminating the irradiation point, since agreat absorption jump in the hemoglobin as well as an absorption drop inthe skin pigment melanin can be detected between these wavelengths, andfurthermore, the varying oxygen saturation of the skin does not have anyinfluence on the measurement. Another advantageous limited spectrumrange for illuminating the skin at the irradiation point lies at about1250 nm, whereby the water content of the tissue essentially determinesthe measurement result at this wavelength.

An embodiment that is characterized in that the scattered light isdetected at several different detection points, at different locations,is very particularly preferred, particularly if the detection points areat a different distance from the irradiation point. Since the intensityof the scattered light decreases with an increasing distance from theirradiation source, according to a specific function, referred to as“scatter function” here, which is dependent not only on the distance butalso dependent on the wavelength of the light used for irradiation,there is the possibility, using this method, of a stringent check as towhether the illuminated sample agrees with natural skin with regard toits scatter function, whereby it is impossible, according to the currentlevel of knowledge, to reproduce this scatter function artificially, inprecise manner, and to additionally provide the material used for anapproximation to this scatter function with the surface profilingnecessary for further reproduction of the papillary line pattern.

If the intensity of the light is modulated over time, so thatalternating light is used, the method is not sensitive to ambientinterference light that cannot be completely eliminated by simpleconventional measures such as shielding, for example.

Equally, there is the possibility that the scattered light is passedfrom the detection point to the detector by way of a light guide.

It is furthermore preferred, within the scope of the invention, if theirradiation point is assigned to a finger or the hand surface of aperson and, at the same time, the characteristic papillary line patternsare detected optically, in contact-free manner, using a papillary linesensor. The great advantage is the further protection against attemptsat deception, since the two measurements for detecting the papillaryline pattern and the scattering behavior of natural skin take placepractically at the same location and almost at the same time, andtherefore there is no possibility of moving or replacing the measurementsample between the two measurements.

The part of the task that relates to the device is accomplished,according to the invention, in that a light source is provided forilluminating the skin surface at the irradiation point, a detector isprovided for detection of the scattered light emitted at the detectionpoint, and a data processing unit, particularly a microprocessor, isprovided as a comparator, and that the light source generates anillumination pattern on the skin that corresponds to an approximate orcomplete circular ring. This structure serves to improve the measurementsecurity by means of forming the average of tissue non-homogeneities andthe generation of sufficiently strong measurement signals, by means ofsuperimposition of a larger number of similar individual scattered lightdistributions.

The illumination pattern that corresponds to the circular ring isgenerated, in simple and therefore preferred manner, in that anillumination ring is assigned to the light source. In the case of theillumination pattern that corresponds to the circular ring, it has beenproven to be advantageous, in view of the greater intensity of thescattered light at a predetermined distance from the entry point, namelythe radius R, if the detection point is assigned to the center of thecircular ring, since the symmetry of illumination is utilized to achievea strong measurement signal for the distance corresponding to the radiusR, in this way.

To determine the scatter function, it is advantageous if several lightsources are arranged in the illumination ring, which emit light atdifferent wavelengths. In this connection, it is advantageous if thenumber of light sources is correlated with a wavelength having thescatter and absorption capacity (scatter function) of the skin at thiswavelength, so that light having a wavelength the scatter function ofwhich leads to a greater attenuation of the intensity at the givendistance, is irradiated in at the irradiation point, by way of theillumination ring having an averaged irradiation intensity, in order tothereby obtain a sufficient measurement signal, which is comparable withthe measurement signals of other wavelengths, with regard to intensity.

Alternatively, there is also the possibility that the diameter of thering-shaped illumination pattern is coordinated with the wavelengthemitted by the light source and the scatter and absorption capacity(scatter function) of the skin, in other words that the intensity of theirradiated light is not varied, but instead the distance of theirradiation point from the detection point is varied, in order toachieve an intensity of the scattered light that can be measured well.

This can be achieved, in particularly simple and therefore preferredmanner, in that at least two illumination rings arranged concentric toone another are provided, which emit light of different wavelengths.

Within the scope of the invention, light-emitting diodes are provided asthe light sources, which represent a circular ring approximated by apolygon, and generate an approximately ring-shaped illumination patternby means of separate light spots. The limitation of the circleapproximation is overcome in that the illumination ring is formed by atotally reflecting tubular piece consisting of an optical material. Thelight can be coupled in on its one side; the light passes through thetubular piece and is reflected multiple times between the walls of thetubular piece, in this connection; in this way, it is uniformlydistributed over the circumference and thereby results in a uniformlyilluminated circular ring having a corresponding illumination pattern.In this connection, the surface on the exit side of the tubular piececan be made rough.

It is furthermore possible that lasers are used as light sources.Furthermore, a lens for imaging the light source on the skin isprovided. In order to reduce the sensitivity of the measurementapparatus to de-focusing, i.e. in order to obtain error-free measurementvalues in a certain depth of definition range, the detector is assignedto a central bore of the lens, in other words a coaxial measurementarrangement is selected.

A compact construction of the device is achieved in that a mirror isarranged in the beam path between the light sources and the lens thatserves to image the illumination pattern on the skin, which mirrordeflects the beam path in such a manner that the lens is arrangedadjacent to or in the interior of the tubular piece.

The generation of an illumination pattern is also possible by means of aconventional projector arrangement, consisting of a light source, acondenser, a projection pattern copy, and a lens assembly, with whichthe irradiation point is illuminated. Furthermore, laser patternprojectors, such as laser diode circle projectors, for example, areadvantageous.

It is also advantageous if a light guide is provided to pass thescattered light to the detector, particularly if the light guide isstructured in flexible manner, and therefore can be guided to thedetector, which is placed at any desired point, even outside of the beampath. If relatively many discrete wavelengths must be used formeasurements in the case of demanding technical solutions forapplication cases with high security requirements, the technicalexpenditure becomes very great, for implementing this with acorrespondingly high number of light sources and illumination rings.According to the invention, a spectrometer, for example a gratingspectrometer of a simple and thereby cost-effective type, is thenarranged in the beam path ahead of the detector, which spectrometerallows a wavelength-dependent evaluation of the scattered lightintensity. It is then advantageous if the detector is formed by aphotodiode array (PDA) or a charged-couple device (CCD). Furthermore, abrightness sensor is provided for monitoring the brightness of the lightsource, for example a monitoring measurement diode, which also providesthe opportunity of constant adjustment.

Within the scope of the invention, it is advantageous if the irradiationsource is assigned to the finger or the inside hand surface of a personand, at the same time, the characteristic line patterns are determinedoptically, in contact-free manner, using a papillary line sensor, sinceintegration of the check for access authorization with the check forattempts at deception takes place in a single device. In particular,there are advantages because the two measurements with regard torecognition as a living being and with regard to biometricidentification, which are to be carried out in contact-free manner, takeplace essentially at the same time, with close spatial coupling of themeasurement regions.

Spatial measurement regions that agree with one another can be achievedif a beam splitter is provided for fade-in of the scattered lightgenerated by the light source and to be measured, into the beam path onthe lens assembly side of the papillary line sensor.

It is also possible and preferred if the detector is formed by thecamera of the papillary line sensor, whereby the camera is provided fortaking pictures of the scattered light reflected by the detection point.When using the camera as a papillary line sensor, it is alsoadvantageous if, in addition to the measurement spot image, a picture ofthe illumination pattern on the skin is also taken, and measured in theimage. In this case, the scattering at the surface of the irradiationpoint that has the volume scatter imposed on it appears as a referencevariable, and stabilization or monitoring of the light source is notabsolutely required.

It is furthermore advantageous, when using the camera of the papillaryline sensor, if a polarization filter is arranged in the illuminationbeam path of the scattered light sensor and ahead of the camera, in acrossed position, in each instance, since gloss effects can be avoidedin the detection of the capillary line patterns in this way. It is apossibility, as well as advantageous, to use the sensor described in DE198 18 229 A1 as a papillary line sensor, since a polarization filter isprovided ahead of the camera here, in any case, and photometricevaluation of the measurement spot image is possible.

A simple and space-saving illumination assembly can be implemented inthat the lens has a central, circular screen assigned to it, and ifseveral light sources are arranged at different distances on the opticalaxis. In this connection, advantage is taken of the fact that a certainlack of focus of the illumination pattern on the skin is not a problem,so that a plurality of ring images is generated by means of thisstructure, in simple manner, whereby the only thing that needs to betaken into consideration is that the screen must be sufficiently largewith reference to the measurement camera, i.e. the detector.

The invention will be explained in greater detail in the following,using exemplary embodiments shown in the drawing; the drawing shows:

FIG. 1 a schematic representation of the procedures when illuminatingskin,

FIG. 2 a graphic representation of the intensity of the scattered light,as a function of the distance r of the detection point from theirradiation point, represented for three different wavelengths,

FIG. 3 a representation, corresponding to FIG. 2, of the superimpositionof two scatter functions of irradiation points arranged at a distance of2 R,

FIG. 4 a schematic representation of the circular ring, with thedetection point arranged in the center,

FIG. 5 a schematic representation of the structure of the deviceaccording to the invention, with light sources of different wavelengthsarranged in different illumination rings,

FIG. 6 a representation comparable to FIG. 5, of an alternativeembodiment that utilizes totally reflecting tubular pieces,

FIG. 7 a representation corresponding to FIG. 5, of another differentembodiment having a light guide and an integrated component for thedetection and evaluation of the scatter function, by means of acomparator,

FIG. 8 a representation corresponding to FIG. 5, with the use of amirror in the illumination beam path, for shortening the structuralshape,

FIG. 9 a simple structure for generating several ring patterns, using ascreen, and

FIG. 10 an alternative arrangement of the light sources and thedetector, for interaction with a device for detecting the papillarylines of a finger.

FIG. 1 shows, in simplified form, how the scattered light 7 is formed,if a light beam 2 having a specific intensity and wavelength is radiatedin at an irradiation point 1. Part of this light beam 2 is diffuselyreflected from the surface of the skin 5, thereby forming the lightbundle 6. The other part of the light beam 2 passes through the surface4 into the tissue of the skin 5, and distributes there by means ofmultiple scattering. A fraction of this light scattered in the skin 5exits back out from the skin surface 4 as visible scattered light 7,whereby the intensity of this scattered light 7 depends, incharacteristic manner, in accordance with the scatter function S, on thedistance of the exit point from the irradiation point 1 as well as onthe wavelength of the light radiated in. This dependence, which issubject to laws and leads to a scatter function S, and is caused by theoptical material properties of the skin 5, is shown schematically forthree different wavelengths in FIG. 2. In order to achieve asufficiently great signal/noise ratio and in order to form the averageover tissue non-homogeneities, the light is irradiated onto the skin 5in an illumination pattern 8 that corresponds to a circular ring, sothat the irradiation point 1 has the shape of this circular ring. Inthis connection, the detection point 9 is assigned to the center of thiscircular ring, as shown in FIG. 4. This illumination pattern in theshape of a circular ring leads to a superimposition of all of thescatter functions which result if the scatter functions shown in FIG. 3are rotated about an axis that goes through half their distance, withthe result that a relatively great intensity is available in the center.

FIG. 5 shows the apparatus structure with which the method for therecognition of natural skin 5 can be carried out, which, in theexemplary embodiment shown, has light sources 10, 11 arranged uniformlyover the circumference of two circles arranged concentric to oneanother, whereby the inner circle has the light sources 10 assigned toit, which generate light having a scatter function S that drops morestrongly, e.g. relatively short-wave visible light, or relativelylong-wave infrared light.

According to an exemplary embodiment not shown in the drawing, it isalternatively possible to arrange the light sources 10, 11 that generatelight of different wavelengths on a single circle, whereby in this case,the numerical ratio of the light sources 10, 11 is adapted to thescatter function S, in other words more short-wave than long-wave lightsources 10, 11 are used for visible light.

The bundled light emitted by the light sources 10, 11, indicated byarrows 12, is directed at a lens 13, which images the light sources 10,11 on the skin 5 as elements 3 of the illumination rings. The scatteredlight 7 exiting from the detection point 9 is measured using a smallmeasurement camera 14, which is located in a central bore of the lens13, whereby the measurement camera 14 consists of the lens assembly 25as well as a detector 20 at the location of the measurement spot image.

The exemplary embodiment according to FIG. 6 differs from the one ofFIG. 5 with regard to the configuration of the illumination. In thisexemplary embodiment, the light sources 10, 11 are directed at the entrysurfaces of two tubular pieces plugged into one another as illuminationrings 15, 16, in which the light is passed to the other side, to theexit surface, on the basis of the total reflection. As an example, thetubular pieces are made of optical material such as acrylic glass. Theexit surfaces are slightly roughened, in order to thereby generate twoapproximately uniformly illuminated ring-shaped sources, which are moreadvantageous, with regard to the measurement accuracy, than ringsegments separated by gaps. FIG. 7 shows the possibility of providingthe detector 20 not at the location of the measurement spot image, butrather at the end of a light conductor 19, in a housing 17, which islocated in the interior of the tubular pieces, whereby this housing 17at the same time also contains the other components that are requiredfor completing the device, such as, for example, the comparator, thepower supply, and the like.

The housing does not necessarily have to be arranged in the interior ofthe tubular pieces. FIG. 8 shows the use of a flexible light guide 19,which leads to a detector 20 that is placed at a different location,fundamentally any desired location. FIG. 8 also shows the use of amirror 21 in the beam path between the light sources 10, 11 and the lens13, in order to thereby achieve a more compact structural shape of thedevice. The arrangement of a spectrometer in the beam path ahead of thedetector 20, in order to allow a wave-dependent evaluation of thescattered light intensity, even without a special selection of the lightsource 10, 11, which can accordingly be selected to be broad-band, isnot shown. The arrangement of a beam splitter for fade-in of the lightgenerated by the light source 10, 11 in the lens-side beam path of thepapillary line sensor 24 is also not shown, whereby the measurements fordetecting the presence of a living being and for identifying a personcan be carried out at the identical position, fundamentally using thesame detector 20, namely the camera 22 of the papillary line sensor 24,which is known from DE 198 18 229 A1 and therefore does not have to bedescribed in detail. FIG. 9 schematically shows the previously knowncapillary line sensor 24, with which “fingerprints,” in particular, canbe determined optically, in contact-free manner. The papillary linepattern is illuminated by means of the light source 10, 11, and detectedusing the camera 22, whereby a polarization filter 23 is arranged bothin the illumination light beam and in the detection light beam, in eachinstance. Depending on the orientation of the polarization filters 23,the pattern of the epidermis or the pattern of the hypodermis can bedetermined; again, reference is made to DE 198 18 229 A1 with regard todetails. In the measurement of the scattered light, only the light thatis scattered with a different polarization is measured, and for thispurpose, the setting of the polarization direction of the polarizationfilters 23 is selected to be opposite in direction. The signal of therecording camera 22 is then also evaluated with regard to the scatteringproperties of natural skin 5.

If one takes into consideration the fact that a certain lack of focus ofthe illumination pattern 8 on the skin is not disruptive, the relativelysimple and space-saving illumination device shown in FIG. 9 can beimplemented.

The light source 10 is imaged on the skin in de-focused manner, by meansof the lens 13, which is provided with a central, circular screen 27,whereby the latter is not larger than the measurement camera 14. Becauseof the active ring-shaped pupil of the lens 13, the de-focused image ofthe light source 10 is also ring-shaped. The second light source 11,having a different wavelength, is positioned on the optical axis infront of or behind the first light source 10, so that a secondconcentric illumination ring is generated. In this way, several lightsources 10, 11 can be arranged on the axis, which generate acorresponding number of ring images. The positions of the light sources10, 11 result from the ring radii selected.

It is true that the lack of focus and the ring width increase with anincreasing ring radius R. However, this can be tolerated, to a greatextent, and can be included in the geometrical considerations relatingto the design of the arrangement. It is advantageous, in terms ofdesign, if positive and negative de-focusing are combined, as far as thedistance between the light sources 10, 11 is concerned.

1. Method for the recognition of natural skin (5) during contact-freebiometric identification of a person, wherein the skin surface (4) isilluminated at an irradiation point (1) assigned to a finger or the palmof the hand, with light from the visible spectrum or the adjacentspectrum, wherein several limited spectrum ranges are used forilluminating the irradiation point, wherein that part of the lightentering through the skin surface (4) at the irradiation point (1),scattered in the skin (5), and exiting from the skin surface (4) again,as scattered light 7, is detected at a detection point (9), using adetector (20) and, at the same time, the characteristic line patternsare determined optically, in contact-free manner, using a papillary linesensor, and wherein the signal determined by the detector (20) is passedto a comparator and compared with stored data.
 2. (canceled) 3.(canceled)
 4. Method according to claim 1, characterized in that lightfrom a spectrum range in which the absorption and/or scattering of anatural substance that naturally occurs in the skin assumecharacteristic values is used for illuminating the irradiation point. 5.Method according to claim 4, characterized in that light from thespectrum ranges around 600 nm and around 800 nm is used for illuminatingthe irradiation point (1).
 6. Method according to claim 5 characterizedin that light from the spectrum range around 1250 nm is used forilluminating the irradiation point (1).
 7. (canceled)
 8. Methodaccording to claim 4, characterized in that the scattered light (7) isdetected at several spatially different detection points (9).
 9. Methodaccording to claim 8, characterized in that the detection points are ata different distance from the irradiation point.
 10. Method according toclaim 1, characterized in that the intensity of the light is modulatedover time.
 11. Method according to claim 1, characterized in that thescattered light (7) is passed to the detector (20) by way of a lightguide (19).
 12. (canceled)
 13. Device for carrying out the methodaccording to claim 1, characterized in that a light source (10, 11) isprovided for illuminating the skin surface at the irradiation point (1),detectors (20) are provided for detection of the scattered light (7)emitted at the detection point (9), a papillary line sensor (24) havinga camera (22) for optical, contact-free determination of thecharacteristic line pattern of the skin (5), and a data processing unit,particularly a microprocessor that functions as a comparator, areprovided.
 14. Device according to claim 13, characterized in that thelight source (10, 11) generates an illumination pattern (8) on the skin(5) that corresponds to an approximate or complete circular ring. 15.Device according to claim 14, characterized in that the light source(10, 11) has an illumination ring (15, 16) assigned to it.
 16. Deviceaccording to claim 14, characterized in that the detection point (9) isassigned to the center of the circular ring.
 17. Device according toclaim 14, characterized in that several light sources (10, 11) arearranged in the illumination ring (15, 16), which emit light atdifferent wavelengths.
 18. Device according to claim 17, characterizedin that the number of light sources (10, 11) having a certain wavelengthis correlated with the scatter and absorption capacity of the skin (5)at this wavelength.
 19. Device according to claim 14, characterized inthat the diameter of the illumination pattern (8) is coordinated withthe wavelength emitted by the light source (10, 11) and the scatter andabsorption capacity of the skin (5) connected with it.
 20. Deviceaccording to claim 19, characterized in that at least two illuminationrings (15, 16) arranged concentric to one another are provided, whichemit light of different wavelengths.
 21. Device according to claim 13,characterized in that light-emitting diodes are provided as lightsources (10, 11).
 22. Device according to claim 13, characterized inthat lasers are provided as light sources (10, 11).
 23. Device accordingto claim 15, characterized in that the illumination ring (15, 16) isformed by a totally reflecting tubular piece consisting of an opticalmaterial.
 24. Device according to claim 13, characterized in that a lens(13) for imaging the light source (10, 11) on the skin (5) is provided.25. Device according to claim 24, characterized in that the detector(20) is assigned to a central bore of the lens (13).
 26. Deviceaccording to claim 24, characterized in that a mirror (21) is arrangedin the beam path between the light sources (10, 11) and the lens (13)that serves to image the illumination pattern (8) on the skin (5), whichmirror deflects the beam path in such a manner that the lens (13) isarranged adjacent to or in the interior of the tubular piece.
 27. Deviceaccording to claim 16, characterized in that the illumination pattern(8) is generated at the irradiation point (1) by means of a projector.28. Device according to claim 13, characterized in that the illuminationpattern (8) is generated by means of a laser pattern projector,particularly a laser diode circle projector.
 29. Device according toclaim 13, characterized in that a light guide (19) is provided to passthe scattered light (7) to the detector (20).
 30. Device according toclaim 29, characterized in that the light guide (19) is structured inflexible manner, and leads to the detector (20), which is arranged atany desired point.
 31. Device according to claim 30, characterized inthat a spectrometer is arranged in the beam path ahead of the detector(20).
 32. Device according to claim 13, characterized in that thedetector (20) is formed by a photodiode array (PDA)
 33. Device accordingto claim 13, characterized in that the detector (20) is formed by acharged-couple device (CCD).
 34. Device according to claim 13,characterized in that a monitor sensor for monitoring the brightness ofthe light sources (10, 11) is provided.
 35. (canceled)
 36. Deviceaccording to claim 35, characterized in that a beam splitter is providedfor fade-in of the light generated by the light source (10, 11) and thescattered light to be measured, into the beam path of the papillary linesensor (24) on the lens assembly side.
 37. Device according to claim 35,characterized in that the detector (20) is formed by the camera (22) ofthe papillary line sensor (24).
 38. Device according to claim 37,characterized in that the camera (22) is provided for taking a pictureboth of the illumination pattern (8) and of the scattered light (7)reflected at the detection point (9).
 39. Device according to claim 35,characterized in that a polarization filter (33) is arranged in theillumination beam path of the scattered light sensor and in thedetection beam path, in each instance.
 40. Device according to claim 24,characterized in that the lens (13) has a central, circular screen (27)assigned to it, and that several light sources (10, 11) are arranged atdifferent distances on the optical axis.